Mortgage Banking Activities - Principal Balance of Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 1,100,000	$ 1,100,000
FHLMC [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	896,553	878,300
FNMA [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 228,074	$ 233,026